Thanks to a study published by Nature Communications, people now know that bats use polarized light to set their internal compass and fly long distances. How the almost blind mammals detect that light is anyone’s guess.
When light hits the earth’s atmosphere it becomes directional, light waves become parallel to each other instead of scattered in all directions. This polarized light creates patterns based on the where the sun is in the sky. These patterns become most evident during sunrise or sunset, in an area across the sky about 90 degrees from the sun’s position.
Many animals use the polarized light patterns to navigate long distances, and as it turns out, bats are one of those animals. Richard Holland of the Queen’s University Belfast explained the motivation behind the study.
“We had already demonstrated that bats used a magnetic compass that was calibrated by cues observed at sunset,” says Holland. “The question was, what cues? It was known that birds calibrate the magnetic field with the pattern of polarization at sunset, so we tried the same for bats.”
Scientists tested the idea by capturing 70 adult female greater mouse-eared bats in their home roost in Bulgaria.
They then placed the bats into holding cages that made the natural polarization of the sun twisted. Those bats were then released with a control group that could see sunlight unobstructed. The bats that went through the polarization treatment veered off from the route the control group took, suggesting that they were lost.
Although this experiment showed that bats did use light to orient themselves, it failed to answer how these animals could see the polarized light.
Fish, birds, reptiles, and amphibians have cone cell structures in their eyes that they use to see the light’s polarization. Bees use special photoreceptors. But bats don’t have any of these physical features.
To add more to the mystery, scientists have also found that bats can even detect polarization patterns well after sunset and on cloudy days.
What about that echolocation, the bat’s answer to sonar?
To hunt and find exact locations, bats use something called echolocation, bouncing sound waves off of nearby surfaces to detect distances and structure. This study’s findings suggest more a combination of senses used in navigation.
As the Holland explains, “We know that bats can use echolocation and vision for navigation when they are in a familiar place or can see familiar cues,” says Holland. “But outside this range the ‘map and compass’ mechanism comes into play, where the animal determines its position and then takes up the compass direction it needs to head in to reach its goal.”
The full study on how bats use light to navigate can be found here.
(Image Credit: Stihler Craig/U.S. Fish and Wildlife Service)